Process and apparatus for cooling milk and other liquids



- PROCESS AND APPARATUS FOR COOLING umc AND 0mm LIQUIDS Filed Sept. 12,.1945

June 13, 1950 v G GRINDROD 2,511,582

2 mvmron 38 Graze: Gem/M00 I Patented June 13,

UNITED STATES PATENT OFFICE rnocass AND APPARATUS FOR oooLmG MILK ANDo'rnaa mourns George Grindrod, Ooonomowoc, Win. Application September12, 1945, Serial No. 615,823

19 Claim. (01. 62-7) My invention relates to improvements in processesand apparatus for cooling milk and other liquids to low temperaturescritically near the freezing point.

My primary object is to obtain a coefllcient of heat transferapproximately three times as great as that heretofore obtained in directexpansion coolers.

More particularly, the objects of my invention are to provide animproved method and improved apparatus for quickly and efllcientlyreducing the temperature of milk and similar liquids to a pointcritically near the freezing point of such liquids without oxidation ordestruction of ascorbic acid and while maintaining a continuous flow ofthe liquid to be cooled through a passage which is closed except atinlet and outlet points, and which has walls at a temperature materiallybelow the freezing point, without danger of coating such walls with iceor clogging the passages; to obtain a rate of heat transfer severaltimes as rapid as that heretofore obtainable in direct expansioncoolers; to provide for automatically and continuousl testing thetemperature and pressure of the circulating liquid and recirculatingsuch portions thereof as may be necessary to ensure its delivery at auniform predetermined low temperature regardless of variations in theinitial temperature or in the volume and velocity of the circulatingliquid.

A further object is to provide a tubular cooler for milk and similarliquids combining the advantages of high power efficiency, maximum heattransfer emciency, protection of the milk from oxidation and loss ofvitamins, maintenance of a more uniform rate of cooling than has beenheretoiore accomplished, particularly while the liquid is approachingthe minimum desired temperature, and regulating the degree ofrecirculation in such a manner as to provide for a continuous andconstant delivery at the desired minimum temperature.

A further object is to provide means whereby a supply of circulatingrefrigerant may be reduced or cut off whenever the circulationof theliquid to be cooled is reduced either by a clogging of the passages or afailure of the supply.

My object is to provide for utilizing an internal tubular cooler withdirect expansion refrigeration, (1) whereby the coefllcient of heattransfer may be made about three times that heretofore obtained indirect expansion coolers,- (2) whereby a higher effective meantemperature difference (M. T. D.) between milk and refrigerated coolingsurfaces may be maintained,

2 and (3) whereby oxidation of the milk or destruction of ascorbic acid(vitamin C) is avoided.

Direct expansion coolers heretofore have been operated at very lowvelocities, the flow of the milk being maintained generally about 1 ft.per second on the assumption that the milk, must stay in contact withthe heat absorbing surface long enough to have its temperature reducedto the desired degree. I have found this theory to be incorrect, mytests having demonstrated that the heat transfer is a function ofvelocity. When using stainless steel tubes of 1% inch diameter and 0.05inch in wall thickness, the heat transfer in terms of B. t. u. persquare foot of heat absorbing surface per minute per degree F. followsthe formula H=0.82V1.3. The velocity (V) is measured in feet per second.According to measurements made in deriving the above relationship, atubular gravity flow cooler of usual design has a transfer co-efllcientof 0.5 B. t. u. A thin corrugated metal direct expansion cooler willgive a coefficient up to about 1.2 B. t. u.- Compared to these theconstant velocity tubular cooler herein described has a co-efflcient of3.0 B. t. u. at its minimum velocity of 5 ft. per second and 7.1 B. t.u. at a velocity of 10 ft. per second of a stream of milk passingcontinuously through the cooler.

The advantage of high mean temperature differenoe is derived from themaintenance of a sustained velocity, high enough to prevent adhesion ofice to the heat absorbing surfaces of the cooler. Under suchcircumstances, the cooling curve approaches the ammonia. temperature asits limit, whereas in the low velocity coolers the cooling curveapproaches 32 degrees as the limit, this being the temperature of theice film. For examplefassume that milk has to be cooled from 60 degreesF. to 34 degrees F., the ammonia suction being maintained at 20 degreesF. For the gravity flow surface cooler the rate of cooling to 34 degreesis as follows:

M. T. 0, -log. a ass For the tubular cooler with sustained velocity ashereinafter described, the rate of cooling to 34 degrees is as follows:

mrnqw-lo gg-uy log a For the tubular cooler hereinafter described,assuming that recirculation will be maintained in a manner to reduce theentering temperature to 47 R, the following valueof M. T. D. willresult:

47 -2o) (34 20 3 MT-Il-W log fl-lg s I The advantage of increasing the.value of M. T. D. is greatest under those conditions where cooling tonear freezing is wanted. Where the refrigerating surfaces cannot bemaintained below ice temperature, the rate of cooling approaches zero asthe temperature 'at the outlet "of the heat exchanger approaches 32degrees F.

This is graphically shown in Figure 4 of the accompanying drawings,which illustrates the hyperbolic curves of cooling."

Cooling of milk to near freezing temperature out of contact with air isparticularly important in connection with the preparation of milk forshipment. When cooled by surface coolers as heretofore, much oxygen isdissolved, the ascorbic acid is destroyed, and the milk become stale.Therefore the only means in commercial practice for complete cooling hasbeen by brine, which is expensive in the use of power.

In the drawings:

Figure 1 is a conventional illustration, in elevation, of a milk coolerembodying my invention.

Figure 2 is a detail view in vertical section showing my temperature andpressure indicator on an enlarged scale.

Figure 3 is a similar detail, showing the recirculating valvemechanisms.

Figure 4 is a time-temperature diagram showing the hyperbolic curves ofcooling under different conditions.

Like parts are identified by the same reference characters throughoutthe several views.

By my improved process, the milk or other liquid to be cooled is drivenin a continuous stream of constant volume and at a scouring velocity incontact with heat absorbing surfaces, access of oxygen orair to the milkbeing prevented. I

The temperature of the heat absorbing surfaces is constantly maintainedat a degree materially below the freezing point of the liquid, whereby arapid heat transfer is maintained after the circulating liquid has beenreduced in temperature to approximately the freezing point.

I have found that by maintaining .the liquid to be cooled at a scouringvelocity of 5 ft. or more per second, flakes of ice tending to form on aheat absorbing surface materially below the freezing point of the liquidare prevented from coating that surface, and are 'scoured therefrom andcarried away in a colloidal condition, the heat absorbing surface beingthus kept clean and free of ice, which would otherwise interfere with,or reduce the cooler and also tend to reduce the capacity of the coolerby tending to obstruct the liquid to be cooled. I also find that theliquid in such motion develops a more rapid rate of heat transfer thancan with liquid in thus enabled relatively slow motion, to quickly bringthe and I am temperature of the milk or other liquid down toapproximately the freezingpoint while circulating it through pipes orpassages, and under such circumstances, I can avoid clogging the pipesor walls thereof.

I provide for automatically. and continuously testing the liquid whichis being cooled, both as to its temperature and pressure. If itstemperathe heat transferring efficiency of- 4 ture has been reduced tothe desired degree it may be delivered to a container or other point ofdischarge, and if not, it is by-passed through the source of supply,mixed with the incoming unchilled liquid and recirculated over therefrigerated surfaces until the temperature of the liquid at the testingpoint reaches the desired be obtained with static liquid or r beingscoured away degree.

Thereafter all of the liquid may be. delivered to the container, or, ifdesired, part of it may continue to be by-passed in sufllcient quantityto reduce the temperature at the heat exchanger inlet to such an extentthat the time required for chilling the liquid to the desired degree maybe correspondingly reduced.

If the temperature of the liquid to be cooled is at any timereducedbelow the freezing point sufllciently to prevent the particles ofice from from the refrigerating sur.- faces, thereby obstructing theflow of the liquid and interfering with heat transfer; or if, for anyother reason, the velocity of the liquid is reduced below that requiredfor scouring flakes of ice from the refrigerating surfaces, therefrigeration of such surfaces is reduced or checked 'until the rate offlow hasagain been restored to said scouring velocity and normal volume.

To facilitate carrying out my improved process, I prefer to use theapparatus disclosed in the drawings. 4

In that apparatus, warm milk is delivered through a supply pipe in to apump ll, usually at a temperature of about 60 degrees F. The pump ll maybe operated by an electric motor l2 at a speed to drive the milk at avelocity of about 5 or 10 feet per second through a pipe l3 which leadsaxially through the expansion chamber of a refrigerating heat exchangerof ordinary type. In the heat exchanger illustrated, a series ofcylinders I, connected by pipes H, are supplied with a suitablerefrigerant at a predetermined low temperature, preferably at such atemperature that the refrigerant in thelast pipe ll of the series can bekept at about 20 degrees F., thus maintaining a wide spread intemperature between the heat of the refrigerant and that of thecirculating milk. Therefore, even in the last pipe section M of theseries a rapid heat transfer may be maintained.

In Figure 1 the motor 20 operates a compressor 2| to place therefrigerant under pressure pre paratory to its liquefaction in acondenser 22 and deliver it to the cylinder H through an expansion valveat 23, the refrigerant returning to the compressor through the pipes 24in accordance with the common practice in refrigerators of the condenserexpander type. A thermostat at 25 is exposed to the refrigerant in thefirst cylinder section I! and operates a suitable switch 25' to controlthe motor circuit and automatically stop the motor .20 when thetemperature to which the thermostat is exposed drops below the desireddegree. In the construction shown, the outside diam eter of the milkpipe I: is 1 inches, and the total length of the expansion chamberrepresented by the pipe sections it should be about i44 feet to providefor a reduction in the temperature of the milk from 60 degrees F. to 32degrees or 34 degrees F., at the calculated rate of heat transfer into arefrigerant circulating under the described conditions. About 16cylinder sections I4, each 9 feet in length, would provide an expansionchamber of 144 feet in length.

The return pipe 24 is provided with a normally closed valve at 28 whichmay be automatically opened by an associated solenoid 21' when thelatter is energized. In the construction illustrated, the valve may beassumed to close downwardly with the pressure of the liquid in thereturn pipe 24, and its stem is connected with a superposed solenoidcore which is lifted toopen the valve when the solenoid is energized.The circuit of the solenoid is controlled by an electric switch 28(Figure 2), mounted on a fitting 2! having a milk passage Ill connectedwith the outlet end or the pipe I I.

At one side 01' the passage to a diaphragm Ii is exposed to the pressureof the milk in the passage, and this diaphragm is connected with aswitch lever 22 controlling the circuit of the solenoid 21, as indicatedin Figure 1. When a normal pressure of the milk in the passage 30 isbeing maintained, the diaphragm 3| will hold the switch 32 in its closedposition. But when the pressure in the passage 30 drops below the nonmai pressure required to maintain a. flow of approximately ieet persecond, the connected end of switch lever 32 will be liited and itsother end depressed by a spring 34 into engagement with the casing, thusdisengaging the switch lever from the contact terminal 33 andde-energizing the solenoid to allow valve 26 to close and stopcirculation of the refrigerant through the ex- 1 chamber or therefrigerator, 1. e., the cylinders i4. Thereupon circulation of therefrigerant ceases. and its temperature is allowed to rise toward normalroom temperature until the rate of flow of the milk through the fitting2! is again restored to the normal or predetermined velocity.

my contact when thediaphragm 41 is depressed It will or course beunderstood that in the construction illustrated the velocity of the milknow through the passage 30 of the fitting 29 is a function of thepressure at which it is delivered to the fitting, and inasmuch as thepressure in the e 30 will be reduced whenever an accumulation of ice onthe inner surfaces of the pipe it reduces the capacit oi that pipe, boththe pressure and rate of flow through the passage it will be reducedaccordingly. A similar reduction in pressure and velocity in the passagell energize the solenoid 21, and allow the valve 26 to close.

As shown in Figure 1, the fitting 29 is connected with another fitting2] by a pipe 38, and the direct passage 39 through the fitting 21 leadsto a delivery pipe to. This fitting, however, has a downwardly extendingoutlet portion 4! conrmigoted by a by-pass pipe 42 with the supply pipeThe passage 39 has a partition at 43, apertured and provided with avalve seat 44 against the partition, as shown in Figure 3. This valve.

may be held against the seat 44 by a valve stem or rod 4, which projectsinto a diaphragm chamber having a diaphragm 4?, urged upwardly by aspring 48. The portion M of the fitting 31 also has ported connectionwith the passage 29 and is provided with a valve seat 52 with whichvalve 4| to move the valve away from the seat 44.

Diaphragm 41 may be depressed by air supplied to the upper portion ofthe diaphragm chamber through a pipe ll connected with a suitable sourceor air pressure supply (not shown). The fitting 28 has an upwardextension II from which a thermostatic air bulb ll depends in contactwith the milk flowing through the passage lll. This air bulb It formspart of an ordinary thermostatic control valve mechanism, hav- I ing avalve at 60 which controls the fiow 01' air through the pipe I! to thediaphragm chamber.

when the milk in passage 30 is too warm ior delivery through the pipe40, the pressure in the bulb 54 will hold the valve in a position tostop the flow of air through the pipe 55. Inthis position the diaphragmchamber above the diaphragm 41 will be vented through the air outlet 82,but as soon as the milk in passage 30 reaches the desired lowtemperature, the pressure in bulb 58 drops and valve 60 moves away fromthe seat 43 and closes upon the seat 64, thus allowing air to flowthrough the pipe 55 to the upper portion of the diaphragm chamber,thereby moving diaphragm 41 downwardly against the pressure of spring 48and carrying valve 45 toward a position closing the .by-pass and openingpassage 39 to delivery pipe 40. These two valves, 44 and ill, operate inalternation to cause either a recirculation of the milk through theby-pass 42 or its final delivery through the pipe 40. It is notnecessary that valve 44 should completely close upon the seat 45. Acollar 49 adjustably threaded upon or otherwise secured to rod 46 may beused to limit the downward movement of valve 44 and thus prevent it fromclosing upon the seat 45. When this is done, a portion of the milk willalways be recirculated.

It will be understood that advantage of compactness is the reason formaking the refrigerating chamber in a series of connected sections l4instead of providing a single long'continuous cylinder. Also, that thelength oi the refrigerating chamber as represented by the combinedlengths of the cylindrical members l4, may be varied in' accordance withthe desired degree of temperature reduction, the size of the milk pipeIS, the thickness or conductivity of its walls and the temperature ofthe refrigerant.

Assuming that the milk at the source of sup ply is at a temperature of60 degrees F., its temperature at the pump [I will be considerablyreduced as soon as recirculation through the by-pass 42 commences.Therefore, if desired, the 'length of the refrigerating chamber may beconsiderably reduced irom that above described, if h y-Dass is neverfully closed and some 01 the milk is being constantly recirculated. Insuch case the suction of the pump-II can be relied upon to draw some,milk through the bypass in all positions of valve 42.

In Figure 4 the line A indicates the hyperbolic cooling curve of a priorart surface cooler in which the refrigerated surface is kept at 32degrees F. It will be noted that after the temperature of the milk hasbeen reduced below 40 degrees, this hyperbolic curve so closelyapproaches the horizontal that the time required to obtain a temperatureof 33 or 34 degrees is greatly prolonged, whereas in curve B thedivergence is not only apparent from the start at 60 degrees ternperature, but the line is nearly straight and the temperature of themilk is reduced to about 32 degrees in about '7 seconds. With 50%recirculation, the milk enters the cooler at about 47 degrees Randleaves it at approximately 32 degrees 1". in less than 4 seconds, asrepresented perature degrees F. r

by-the hyperbolic curve C, the ammonia tembeing maintained atapproximately 20 8. tionsofsaidliquidtomaintainarapidheatex changethroughout the cooling interval, and varying the rate of flow of therefrigerant in proportion to variations in the pressure and velocity ofsaid stream as it passes out of heat exchanging proximity to therefrigerant.

The dotted lines B'-C' extending the curves velocity regardless of thevolume delivered from the source of milk suppl The minimum velocitywhich will prevent freezing of ice film in a 1% inch stainless steeltube has been found tobeabout5 ft. per second. Ifthe milk isbeing cooledto 34 degrees l". by ammonia as the refrigerating agent, maintained at20'degrees F.

in contact with the exterior surfaces of such tubing, at avelocity ofabout 5 ft. per second, 12,000 poundsof milk per hour may be pumpedthrough such a 1% inch tube, the internal cross sectional area of whichis 1.54 square inches. At a velocity of ft. per second, the heattransfer coeflicient is more than doubled. At that rate, 22,000. poundsper hour may be pumped pounds per hour, the M. T. D. may-be reduced from24.8 degrees to 19.8 degrees, but the heat tl'gI-nsfer coefllcient wouldbe increased from 3 B. t. u. to 7.1 B. t. u. Thus, the net increase incooler capacity would be represented by the ratio:24.8x3:19.8x'l.1='l7.4:140.'l=1:1.87.

This would be at the-expense of greater power of pumping. If the volumeof milk to be cooled is les than that required to give at least 5 ft.per second, then recirculation is required. Otherwise the milk wouldfreeze unless the temperature of the refrigerating ammonia is raised. Ifmore than 5 ft. per second is obtained by the norflow of the milk, thenrecirculation may not required to obtain the desired reduction in butthe capacity would be increased.

2. The process of rapidly cooling milk and other liquids from 'atemperature of about 80 F. to a temperature of about 32 F., consistingin piping the liquid under pressure and at a velocity while maintainingthep pe at a temperature sufiiciently below thefreezing point of theliquid to maintain a rapid heat exchange while Blncethe power requiredto pump at 10 ft. per

is over four times that required to pump at 5 ft. per second, there isordinarily no advantage obtained in maintaining a rate of flow muchabove the minimum. Accordingly, theusual practical working range ofvelocity is at a point somewhere between 5 ft. per second and 10 ft. persecond, provided the ammonia suction is maintained at approximately 35lb. gauge. If the ammonia compressor has sufllcient capacity, thecapacity of the cooler may be greatly increased by reducing the ammoniapressure and simultaneously increasing the velocity and recirculation.'lhis cooler therefore has the advan-- tage of great flexibility ofcapacity, together with highheat transfer emciency at or near thefreezing point. 1

I claim:

1. The process of rapidly cooling milk and other liquids to a degreeslightly above their freezing point, consisting in forcing the liquid'ina conflned stream and at a scouring velocity in heat exchangingproximity to a flowing refrigerant having a temperature below thefreezing point of said liquid and materially below the ultimatetemperature to which such liquid is to be reduced, whereby to maintain asubstantial temperature diiicrential between the refrigerant and allnotthe liquid is approaching its freezing'point, mixing some of thecooled liquid with that coming from the source of supply to effect animmediate reduction. in the temperature of the inflowing stream, andraising the temperature of the pipe when the pressure and velocity ofthe cooled liquid falls below that at which the liquid is normallydelivered from the pipe. I

3. The process of rapidly cooling milk to a temperature near thefreezing point of the milk,

which consists in causing the-milk to flow in a confined stream and at ascouring velocity in heat exchanging proximity to a circulating streamof refrigerant until the milk is reduced in temperature to about 32 F.,normally maintaining the temperature of the refrigerant at a temperaturegreatly below the ultimate temperature of the milk and substantiallybelow the freezing point of the milk, reducing the rate of flow of therefrigerant when the pressure of the milk in the conflned stream leavingthe refrigerant drops materially below the pressure at which it wasdelivered into heat exchanging proximity to the refrigerant. I

4. Means for rapidly cooling liquid from a temperature of about 60 F. toa temperature of about 32 F., comprisingin combination, a heat exchangerhaving a pipe for liquid to be refrigerated and a Jacket for arefrigerant at a temperature materially below the freezing point of saidliquid', means for forcing liquid through the pipe at a scouringvelocity, a flexible diaphragm exposed to the pressure of the liquid inthe outlet portion of said pipe, a spring for urging the diaphragminopposition to the pressure of the liquid thereon, refrigerantconnections to the jacket including a valve, and means connecting thediaphragm, with the valve for regulating the delivery of l'efflBef? antto the heat exchanger.

5. Means for rapidly cooling liquid from a temperature of about 60 F. toa temperature of about 32 F., comprising in combinations heat exchangerhaving a pipe for such liquid and a Jacket for refrigerant at atemperature materially below the freezing point of said liquid, meansfor forcing liquid through the pipe at a scouring velocity, a flexiblediaphragm exposed to the pressure of the liquid in the outlet Portion ofsaid pipe, a spring for urging the diaphragm in opposition to thepressure of they liquid thereon, re-

. -frigerant connections to the Jacket including a low the freezingpoint of such liquid, a heat exo v 1 W s pipe for circulating liquidthrough the heat exchanger, means for delivering liquid through saidpipe at a scouring velocity, a flexible diaphragm exposed to thepressure of the liquid in the outlet portion of the pipe, a spring forurging the diaphragm in opposition to the pressure of the liquidthereon, a valve having actuating connections controlled by thediaphragm for tying the delivery of refrigerant to the heat exchanger, aby-pass connecting the outlet end portion of said pipe to the inletportion, a thermostatically controlled valve in said by-pass forregulating the flow of liquid therethrough, and means for preventing thevalve from moving to a completely closed position, whereby some of thedifferential between the liquid and such surface cooled liquid iscontinuously mixed with that coming from the source of supply toinstantly reduce its temperature.

I. In a milk cooler having a heat exchanger for maintaining a rapid heatreduction from the milkwhile its temperature is approaching the freezingpoint, the combination with a milk conveying pipe extending through theheat exchanger, of a fitting in the outlet portion of the .pipe having adiaphragm subject to the pressure of the milk, a valve controllingdelivery ofrefrigerant to the heat exchanger, a solenoid for opening thevalve, a switch operatively connected with the diaphragm for controllingan electric circuit through the solenoid, and means for exerting acounter pressure on the diaphragm to move the switch to open positionwhen the milk pressure in the fitting drops below normal.

8. in a milk cooler having a heat exchanger for maintaining a rapid heatreduction from the milk while its temperature is approaching thefreezing point, the combination with a milk conveying pipe extendingthrough the heat ex-,

changer, of a fitting in the outlet portion of the pipe having adiaphragm subject to the pressure oi the milk, a valve controllingdelivery of refrigerant to the heat exchanger, a solenoid for openingthe valve, a switch operatively connected with the diaphragm forcontrolling an electric circuit through the solenoid, means for exertinga counter pressure on the diaphragmto move the switch to open positionwhen the milk pressure in the fitting drops below normal, said fittingbeing provided with a thermostat, and means controlled by the thermostatfor by-passing some of the milk for recirculation through the heatexchanger.

9. In a milk cooler-having a heat exchanger provided with means fordelivering a refrigerant thereto at a temperature materially below 32 F.and a pipe system for conveying milk through the heat exchanger andhaving a milk delivering portion, said system including a valved by-passfor recirculating some of the milk through the heat exchanger until thetemperature of the milk approaches the freezing point, the combinationtherewith of a thermostat connected with the bypass valve forcontrolling circulation through the by-pass, means controllingrefrigerant delivery to said exchanger, and pressure responsive meansmounted in the said delivering portion and connected to said refrigerantcontrolling means for checking deliveries of refrigerant to the heatexchanger when the milk pressure in the fitting drops below normal.

10. In a milk cooler of the described class, the combination with meansfor circulating and recirculating milk through a heat exchanger at ascouring velocity, of a fitting through which the milk is deliveredafter being cooled ln'the heat exchanger, said fitting having athermostat rebeing sufiiciently great to effect a nearly rectilinearcurve of temperature drop of the liquid to the desired temperature, thevelocity of fiow of the liquid keeping the liquid from freezing on suchsurface, and mixing a first component of the liquid cooled in traversingsuch surface with a second component of liquid previousto its traverseof such surface whereby to eflect rapid initial cooling of said secondcomponent and to maintain a desired scouring velocity of fiow and rate,of heat exchange between such liquid and such surface.

12. A method of rapidly cooling milk to a temperature in proximity toits freezing point, such method consisting in passing the milk at ascouring velocity of at least approximately five feet per second througha heat exchange tube, refrigerating the tube to a temperature materiallybelow the freezing point of the milk, and removing the milk from therefrigerated tube at a temperature materially above the temperature towhich the tube is refrigerated, whereby the milk is at all times duringcontact with the refrigerated tube subject to a substantial temperaturedifferential which efiects rapid cooling of the milk, the rate of milkfiow through the tube precluding the freezing of milk thereto, togetherwith the further step of recirculating a portion of the milk which hastraversed such tube in admixture with a portion of the milk which hasnot yet traversed such tube, whereby to reduce the temperature of thelatter milk portion and to reduce the temperature differential betweenthe total body of milk and the tube .while compensating for such reduceddifferential by accelerating heat exchange in consequence of theincreased velocity required for such recirculation.

13. A method of rapidly cooling a liquid to a temperature near itsfreezing oint which comprises forcing the liquid in a. confined streamand a scouring velocity across a heat exchange surface having asub-freezing temperature, and recirculating a portion of the liquid intocontact with portions of such liquid which have not yet traversed saidsurface whereby to eflect instant cooling of said last mentionedportions and to augment the rate of fiow over said surface.

14. The method recited in claim 13 wherein the amount of liquidrecirculated is varied in proportion to the temperature of the liquidleaving such surface.

15. A method of rapidly cooling liquid, which method consists in passingthe liquid over a heat absorbing surface in a continuous stream,diverting a portion of said stream which has traversed such surface, andrecirculating the diverted portion of said stream over said surface inadmixture with another portion of said stream which has not previouslytraversed said surface.

16. The 'method recited in claim 15 in which the volume of the divertedportion of the stream is varied in proportion to the temperature 11 ofthe stream after being acted upon by said surface.

17. Apparatus for rapidly reducing the temperature of a liquid, saidapparatus comprising a heat exchanger having a pipe for the liquid to becooled, a Jacket about the pipe for refrigerant, a pump connected withthe pipe for circulating therethrough the liquid to be cooled, and a bypass for recirculation of a portion of the cooled point of the liquid tobe cooled. and said pump f:

being adapted to force liquid through said pipe at scouring velocities.sunicient to preclude the freezing of the liquid on the pipe, saidvelocity being increased by the volume of liquid recirculated.

19. The device of claim 17 in which the said Jacket is provided withrefrigerant supply con motions and refrigerant flow controlling meansfor maintaining the jacketed portion of the pipe at a temperaturematerially below the freezing point of the liquid to be cooled, and saidpump being adapted to force liquid through said pipe at scouringvelocities sufllcient to preclude. the

freezing of the liquid on the pipe, said velocity being increased by thevolume of liquid recirculated, together with a thermostaticallycontrolled valve regulating recirculation.

GEORGE GRINDROD.

' REFERENCES CITED The following references are of record in the file ofthis patent:

V UNITED STATES PATENTS Number Name Date 2,107,053 Coons Feb. 1, 19382,316,845 Craft Apr. 20, 1943 2,389,106 Marshall Nov. 13, 1945 MurrayDec. 30, 1947

